Some questions on maintaining addons with multiple Blender versions
Blender saves its config files in a different folder for each version of blender. If your custom build is based on 2.79 then it will read the same config as the standard 2.79 build. The location and layout of config folders is explained here.

To have the same addon available to multiple versions, you need a copy of the addon in each versions config folder. To simplify upgrading you can use links to have each version read the same file contents. Links are also known as shortcuts or aliases. Create a folder for your addons and put your addons there, then create a shortcut in each config folder that you want the addon to be available. This may be easiest if you install the addon for the current version and then make links from that to any older versions you want to use the addon.